This invention relates to a display control device and a display control method for displaying a plurality of pictures with one picture being superposed on another on a display screen and, more particularly, to a device and a method of this type suitable for displaying a number of pictures without increasing an amount of transfer of data to be displayed.
In consumer type display control devices such as one for a game machine, a picture to be displayed is divided into parts and these parts are allocated to different display positions on a display screen and depth of the picture to be displayed is expressed by controlling the amount of scroll for each part and a priority order of superposed parts. For example, parts such as "clouds in the sky", "a mountain in the distance", "a road extending laterally in the picture and buildings along the road" and "an automobile running on the road" are allocated to different display positions. A part existing in the distance is given a lower priority order and a smaller amount of scroll than a part existing in a near position whereby depth in the whole picture can be expressed.
In this type of display control device, it is desirable to set a very fine priority order for respective parts each having a very narrow display zone allocated in the display screen. For this purpose, a prior art display control device adopts a system in which video data for all parts of a picture is transferred from a video memory and the picture to be displayed is reconstructed in accordance with a priority number attached to the respective video data. This system has the advantage that, since a preferential processing is performed after video data has been transferred from a frame buffer, the priority order can be controlled dot by dot by setting, for example, a color code of transparent display (i.e., displaying video data of a part having a lower priority order).
On the other hand, a video accelerator adapted for WINDOWS (trademark of Microsoft Inc.) which is used as a current main personal computer operating system adopts a system in which, in a case where display zones of windows set for respective applications overlap each other, video data of the overlapping display zones is rewritten to shift a lower window to the highest priority and superposition of windows can thereby be realized with only a single video data stored in the video memory. In this system, however, the video memory must be rewritten each time shifting of a window takes place so that there arises the problem that a smooth shifting of a window in real time cannot be made. In this system, therefore, a window frame only is written during shifting of a window for reducing the amount of data to be written and video data in the window is written after the location of the shifted window has been established.
In the former prior art display control device, video data of all parts of a picture must be transferred and, for achieving this, data bus of a broad width must be employed or a high speed memory must be used for increasing the amount of data to be transferred. The increase in the data bus necessitates parallel connection of plural frame buffers resulting in increase in the number of pins provided in an interface with the display control device which increases the manufacturing cost of the device. In the latter prior art display control device, there frequently occurs a case where writing of video data cannot catch up with shifting of a window of a large display zone or quick shifting of a window.
It is, therefore, an object of the invention to provide a display control device which enables display of plural pictures on a display screen and a smooth shifting of windows without increasing the amount of transferred data.